"The attack on the part [Mosul] to the east of the Tigris River was primarily undertaken by the highly trained and experienced Counter-Terrorism Service (CTS), fighting house to house. Air strikes were usually against carefully selected targets, and not called in at will by ground troops at the first sign of resistance."

"These tactics of the pro-government forces did not work. True, they eventually captured east Mosul after three months of heavy fighting and at the cost of casualties to the CTS reported as being between 40 and 50 per cent. But they could not afford this scale of losses repeated in west Mosul, where Isis was even more deeply entrenched."

"When the assault on west Mosul began on 19 February, the pro-government forces were therefore using artillery, rockets and air power much more freely. And in addition to the CTS, they fielded the Federal Police and Emergency Response Division, both of which were far less well-trained and deemed more sectarian than the CTS. As they in turn suffered heavy casualties, they lost all restraint in use of their firepower."

There you have it. Counter Terrorism Service troops well-trained and prepared for urban warfare less reliant on heavy firepower and able to use a nuanced approach but still sustaining heavy casualties.

Less well-trained troops during a period of prolonged urban warfare becoming too reliant on heavy firepower and using same in a more indiscriminate manner.